Polished rod blowout preventers (rod BOPs) have been in use for oil and gas wells for over fifty years. Known types of rod BOPs have incorporated a variety of different ram designs, but they have typically included a large elastomeric seal section with a small steel insert to support it.
As working pressures at the wellhead increased, new rod BOP designs came into common use which had opposing rams made of steel, with each ram having a groove into which an elastomeric seal element could be installed. In such designs, the seal groove on each ram extends transversely across the inner end face of ram, axially along the side of the ram and then along a circular path around at least a portion of the circumference of the ram, thereby creating a seal between the rams and their respective bores, and also creating a seal between each set of opposing rams and the polished rod. The seals have a cross-section that is smaller than the grooves, thereby allowing them to be compressed into the grooves. This design provided enhanced sealing effectiveness at higher pressures, as compared with earlier polished rod BOP seal designs. One example of this type of ram seal for a rod BOP can be seen in Canadian Patent No. 2,716,430.
In recent years, however, the process of hydraulic fracturing (commonly referred to as “fraccing”) has added a new consideration to design and performance criteria for polished rod BOPs. Wells are often situated in close proximity to each other, and during fraccing operations in subsurface formations penetrated by multiple and relatively closely-spaced wells, fluid communication between wells has become common. Due to such fluid communication, high operating pressures in one well can result in similarly high pressures undesirably developing in one or more adjacent wells, which may necessitate closure of the BOPs on those wells.
However, for wells having prior art BOPs of the type described above, when the rams with narrow grooves are closed, a problem occurs: as the aperture between the polished rod and the opposing rams gets smaller, the velocity of the wellbore fluid passing through the BOP increases rapidly. The resultant high fluid velocity causes displacement or “extrusion” of the seals out of their grooves in the rams, resulting in failure of the BOP at exactly the time when well control is most required.
For the foregoing reason, there is a need for improved polished rod blowout preventers that will function in conditions of concurrent high fluid pressure and high fluid velocity, while reducing or eliminating the risk of extrusion of the ram seals and resultant loss of BOP effectiveness.